Fibre-containing composite materials such as CFRP and GFRP represent a particular challenge, especially in terms of good hole quality. Furthermore, in the aerospace industry especially, these materials are often drilled by hand, rather than by automated drilling because the workpiece (e.g. component or panel) may not be accessible to the arm of an automated drilling apparatus.
Unlike automated drilling where the drilling is computer controlled (i.e. drill speed and drill feed rate), hand drilling requires the user to control the speed and feed. Thus, whilst damage to the workpiece (e.g. composite material) can be moderated in the case of automated drilling by control of drill speed and feed rate, in hand drilling, damage to the workpiece is dependent on the operator, namely the speed and feed applied to the workpiece. Hence, hand drilling brings with it the difficulty of producing uniform holes: different users may apply different levels of force to the workpiece and so hole quality can vary.
Hand drilling is also more susceptible to the problem of “pushing” or “pull through”, whereby a drill can exert an axial force on the user at particular points during drilling of a hole. This arises from the interaction of the drill and the workpiece as the drill moves into and through the workpiece. This makes it difficult for the user to control the drilling process and can lead to reduced hole uniformity and hole quality.
Commercially available drills that are suggested for use with CFRP-type materials and the like are drill reamers, particularly four-flute drill reamers. However, these are unable to provide a variable level of thrust force and good exit hole quality.